1240815StormWaterManagementReport

STORMWATER MANAGEMENT As with any construction project, stormwater management is an important part of the overall development process. Modern practice is a combination of the site designer’s knowledge and goals in conjunction with the rules and regulations of the local municipality and the New Jersey Department of Environmental Protection (see Hydrology section below). Part of stormwater management is to minimize the use of artificial structures; however, practical considerations of most development sites require a combination of non-structural and structural components. A substantial portion of the existing site contains buildings and bituminous pavement. The entire site drains to the south area where it runs thru a swale to the street on the rear and is collected by the storm system within Beasly Street. Stormwater hydrographs for both existing and proposed conditions for each drainage area, are included in this report. The Township of West Orange requires that stormwater analyses be performed for the 2-year, 10-year, and 100-year storm events, applying reduction factors of 50%, 25%, and 20%, respectively, for major developments. Although this project does not qualify as a major development, these reductions were voluntarily applied to further enhance site performance. The “Natural Resources Conservation Service Method,” as outlined in Technical Release 55 (TR-55), was utilized to size and route stormwater through the proposed drywell system. DISCHARGE SUMMARY The following table shows that the site would produce post-developed runoff rates and volumes that are lower than the site’s existing rates of runoff. The existing and proposed peak runoff rates (Q) from the site are summarized as follows: Site Discharge Summary Peak Detention Flow Subject to Q Q Total Q Frequency Reduction Reduction Existing Allowable Proposed Q Non Disturb 2 - 2020 0.98 0.50 + 0.00 = 0.49 0.43 10 - 2020 1.63 0.25 + 0.00 = 1.22 0.70 100 - 2020 2.97 0.20 + 0.00 = 2.38 1.89 The Site Discharge table shows that the proposed project would result in post-developed runoff rates that are less than or equal to

7 0.20 + 0.00 = 2.38 1.89 The Site Discharge table shows that the proposed project would result in post-developed runoff rates that are less than or equal to the site’s existing runoff rates. The hydrographs for the 2-year, 10-year and 100-year storms do not exceed at any time the pre-developed hydrographs for the same events as required per West Orange’s ordinance. Based upon this analysis, it is our professional opinion that the proposed storm water management system would enhance the existing downstream conveyance systems and meet the required Municipal standards. Calculations supporting these findings constitute the remainder of this report. 2